1. Field of the Invention
The present invention relates to a resin double helical gear pair composing part of a gear train for power transmission.
2. Description of the Related Art
In the past, a gearing resin double helical gear pair has been used as part of a gear train for power transmission, in order to quiet down the operating sound and deaden the vibrations in power transmission (refer to Patent Literature 1).
However, in the gearing resin double helical gear pair, a larger stress acts on a dedendum of a peak located in the center in the full tooth width direction of a chevron tooth, compared to a stress on a dedendum of sections other than the vicinity of the peak of the tooth. Therefore, the gearing resin double helical gear pair firstly comes to the end of its life due to the resultant stress difference.
Therefore, engineers taking charge of mechanical structure design have demanded a technology by which the stress value of the dedendum of the peak located in the center in the full tooth width direction of the resin double helical gear can be reduced, and by which the strength of the tooth of the resin double helical gear can be increased.
Specially, in the case that the rotational speed of one resin double helical gear is different from that of the other resin double helical gear in a resin double helical gear pair (in the case of deceleration or acceleration), the resin double helical gear with the higher rotational speed comes to the end of its life earlier than the resin double helical gear with the lower rotational speed.
For example, when a double helical gear with the smaller diameter and a double helical gear with the larger diameter are combined in a gearing resin double helical gear pair, a large stress acts on the dedendum of the vicinity of the peak of the tooth in the double helical gear with the smaller diameter and the higher rotational speed, and thus the double helical gear with the smaller diameter comes to the end of its life first.
Therefore, engineers taking charge of mechanical structure design have demanded that the tooth of the resin double helical gear with the higher rotational speed is reinforced, and thereby the life of the resin double helical gear with the higher rotational speed and the life of the resin double helical gear with the lower rotational speed are approximated to each other.
Further, experience shows the following fact. That is, even when 2 double helical gears with the identical diameter gear into each other in a gearing resin double helical gear pair, the dedendum of the vicinity of the peak of a tooth of the double helical gear on the drive side comes to the end of its life earlier than the dedendum of the vicinity of the peak of a tooth of the double helical gear on the driven side.
As a method to solve the forgoing problem, it is conceivable that the tooth thickness of the dedendum of a tooth is thickened by addendum modification, and thereby the dedendum strength is increased (refer to Nonpatent Literature 1).
A technology to change the addendum modification coefficient of a tooth of a spur gear from one end side to the other end side in the tooth width direction has been already developed (refer to Patent Literature 2).
However, in the existing technology disclosed in Nonpatent Literature 1, the entire tooth is provided with positive addendum modification. Therefore, when such a technology is applied to the resin double helical gear, the tooth thickness of the dedendum of the tooth is thickened uniformly in the entire region in the tooth width direction, and thus it is not possible to effectively reinforce only the dedendum of the vicinity of the peak of the tooth necessary for reinforcement. In result, the entire resin amount is increased, and thus the entire weight becomes excessively heavy. Further, the gearing ratio is decreased, and thus the noise is increased.
Further, the existing technology disclosed in Patent Literature 2 is intended to adjust backlash of a gearing section to inhibit generation of abnormal sound resulting from the backlash. In such a technology, the addendum modification coefficient is changed from one end side to the other end side in the tooth width direction. Therefore, it is not possible to thicken the tooth thickness of the dedendum in the section on which a stress is concentrated more than the thickness of the dedendum in the other sections and to effectively increase the strength of the dedendum.
Patent Literature 1: Japanese Patent Laid-open Publication No. 2005-69458 (refer to paragraph No. 0007 and FIG. 4)
Patent Literature 2: Japanese Patent Laid-open Publication No. 2006-250182 (refer to paragraph No. 0020 and FIG. 3)
Nonpatent Literature 1: “Machine design: design of mechanical components and system,” Shigeka Yoshimoto et al., First edition, Rikogaku Co., May 20, 2006, p. 208.